Forces in Action

Types of forces, free body diagrams, moments, density and pressure.

Spec Points Covered
  • I can identify and label all forces acting on an object: weight, normal contact, friction, tension, drag and upthrust.
  • I can draw accurate free body diagrams with correctly scaled and labelled force arrows.
  • I can state Newton's first law and apply it to objects in equilibriumAn object is in equilibrium when the resultant force on it is zero. The object is either stationary or moving at constant velocityThe rate of change of displacement. A vector quantity. Measured in m s⁻¹.. and at constant velocityThe rate of change of displacement. A vector quantity. Measured in m s⁻¹..
  • I can explain terminal velocityThe rate of change of displacement. A vector quantity. Measured in m s⁻¹.The constant velocity reached when the driving force on an object is exactly balanced by resistive forces, so the resultant force is zero. in terms of the balance between weight and drag.
  • I can calculate the moment of a forceThe turning effect of a force about a pivot. Equal to force multiplied by the perpendicular distance from the pivot to the line of action of the force. about a pivot using the perpendicular distance.
  • I can apply the principle of momentsFor an object in rotational equilibriumAn object is in equilibrium when the resultant force on it is zero. The object is either stationary or moving at constant velocity., the sum of clockwise moments about any point equals the sum of anticlockwise moments about the same point. to solve equilibriumAn object is in equilibrium when the resultant force on it is zero. The object is either stationary or moving at constant velocity. problems.
  • I can distinguish between a coupleA pair of equal and opposite forces whose lines of action do not coincide, producing a pure turning effect (torque) with no resultant force. and a single moment, and calculate the torque of a coupleA pair of equal and opposite forces whose lines of action do not coincide, producing a pure turning effect (torque) with no resultant force..
  • I can explain the conditions for equilibrium: zero resultant force and zero resultant moment.
  • I can use $densityMass per unit volume of a material. Measured in kg m⁻³. = mass/volume$ and $pressureForce per unit area. Measured in pascals (Pa), where 1 Pa = 1 N m⁻². = force/area$ in calculations.
  • I can derive and apply $p = \rho g h$ for pressureForce per unit area. Measured in pascals (Pa), where 1 Pa = 1 N m⁻². in a fluid column and explain Archimedes' principle.
Notes
01 Weight (W = mg) → 02 Newton's First Law Newton's First Law → 03 Terminal velocity Terminal velocity → 04 Moment of a force Moment of a force → 05 Couple Couple → 06 Centre of mass Centre of mass → 07 Density Density → 08 Pressure Pressure →
Σ Key Equations Full Reference →
On Data Sheet
Not on Data Sheet
Moment of a force
$$\text{moment} = F \times d$$
  • Where:
    • $F$ = N
    • $d$ = m
    • $moment$ = N m
d must be the perpendicular distance from the pivot to the line of action of the force.
Torque of a couple
$$\tau = F \times d$$
  • Where:
    • $F$ = N
    • $d$ = m
    • $\tau$ = N m
d is the perpendicular distance between the lines of action of the two equal and opposite forces.
Pressure
$$p = \frac{F}{A}$$
  • Where:
    • $p$ = Pa (N \(m^{-2}\))
    • $F$ = N
    • $A$ = \(m^{2}\)
F must be the force acting perpendicular to the surface of area A.
Density
$$\rho = \frac{m}{V}$$
  • Where:
    • $\rho$ = kg \(m^{-3}\)
    • $m$ = kg
    • $V$ = \(m^{3}\)
Density is a material property. Convert cm^3 to \(m^{3}\) by multiplying by \(10^{-6}\).
Pressure in a fluid column
$$p = \rho g h$$
  • Where:
    • $p$ = Pa
    • $\rho$ = kg \(m^{-3}\)
    • $g$ = m \(s^{-2}\)
    • $h$ = m
h is the vertical depth below the surface. This gives only the pressure due to the fluid; add atmospheric pressure for total pressure.
Weight
$$W = mg$$
  • Where:
    • $W$ = N
    • $m$ = kg
    • $g$ = m \(s^{-2}\)
Weight is the gravitational force on a mass. Use g = 9.81 m \(s^{-2}\) unless stated otherwise.